Polyimide-based X-ray masks with advanced performance of pattern accuracy and thermal stability

Polyimide-based X-ray masks which are generated by optical lithography and electroplating of gold absorbers on a polyimide mask membrane gain attention as low cost masks. The organic membranes generally have problem of (1) pattern edge sharpness, (2) miniaturization of pattern and (3) thermal stability. To make the masks commodity in deep and accurate lithography area, Optnics Precision, Japan, has overcome the above difficulties and realized the masks with advanced performance of pattern accuracy and thermal stability by improving the making process and the material. Good results were obtained in the exposure experiment that used this mask. When this X-ray mask is combined with the electroforming technique and the material development technique that Optnics has, the application to various fields like an industrial field and medical field can be expected.     

Fabrication of intermediate mask for deep x-ray lithography

 This paper presents the fabrication of intermediate x-ray mask for deep x-ray lithography. In order to have working mask with absorbers thickness larger than 10 μm, the intermediate mask should have absorbers of 0.7 μm in thickness. To demonstrate intermediate mask fabrication, x-ray zone plates are fabricated on the 1.2 μm low-stress silicon-rich silicon nitride (SiN_x) membrane with the tri-layer Chromium-Tungsten-Chromium (Cr–W–Cr) as the x-ray absorbers. The chromium layers both 200 angstroms are used as adhesion and for stress relief. The SiN_x film is deposited with low pressure chemical vapor deposition (LPCVD) and the free standing membrane are formed by KOH silicon backside etching. With the e-beam lithography and reactive ion etching, width of 0.8 μm of outmost zone of the x-ray zone plates has been achieved on the membrane. The scanning electron microscopy (SEM) images of the x-ray zone plates and pictures of intermediate masks are demonstrated. Received: 25 August 1997/Accepted: 3 September 1997     

Fabrication of sub-micron structures for MEMS using deep X-ray lithography

Fabrication techniques of microstructures with high resolution and high aspect ratio are necessary for practical microelectromechanical systems (MEMS) that have high performance and integration. In order to fabricate microstructures with sub-micron resolution and high aspect ratio, deep X-ray lithography has been investigated using the compact synchrotron radiation (SR) light source called “AURORA”. An X-ray mask for sub-micron deep X-ray lithography, which is composed of 1 μm thick Au as absorbers, 2 μm thick SiC as a membrane and 625 μm thick Si as a frame, was designed. In preliminary experiments, the following results were achieved: EB resist microstructures with an aspect ratio of 22 corresponding with 0.07 μm width and 1.3 μm height were formed; a 10 μm thick PMMA resist containing no warp was formed by direct polymerization, enabling more precise gap control.     

The fabrication of LIGA mask using photolithography and synchrotron radiation lithography

 The fabrication of LIGA mask is a very important step in LIGA process. Usually an intermediate mask with gold absorber pattern of 2 μm thickness is fabricated firstly using gold electroplating for absorber pattern in the resist structure written by Electron Beam (e-Beam), then the LIGA mask can be copied from the intermediate mask using synchrotron radiation lithography and gold electroplating. Recently, we use photolithography (instead of e-beam) to make the primary structure, and produce the intermediate mask with gold absorber pattern of 1.5 μm thickness produced by etching gold film with 1.5 μm thickness under the photoresist structure using Ar⁺. The LIGA mask with absorber pattern of 13 μm thickness is copied from the intermediate mask using synchrotron radiation lithography and gold electroplating. Received: 30 October 1995/Accepted: 9 December 1995     

Study on fabrication of high aspect ratio MEMS microparts using a compact SR beamline

 A compact beamline dedicated to hard x-ray deep lithography for fabrication of high aspect ratio MEMS microparts has been developed. The exposure stage was only 3 meters away from the synchrotron radiation (SR) source so that a relatively high photon flux could be achieved with a compact super-conducting SR source. The deep lithography using PMMA resist could be as deep as 1000 μm and the maximum aspect ratio achieved was about 50. The throughout for the 200 μm-deep lithography was found to be on the order of 5 cm²/h using the membrane-free mask under the routine SR conditions. Templates with the high aspect ratio microstructures have been made of the PMMA resist based on conducting substrates and applied further to electroforming to create metallic microstructures. In order to fabricate microparts for the MEMS applications, we have concentrated on development of masks for the hard x-ray deep lithography. The masks now can be made to have the 8 μm-thick gold absorber on the 2 μm-thick SiC membrane. Received: 25 August 1997/Accepted: 3 September 1997     

Process conditions in X-ray lithography for the fabrication of devices with sub-micron feature sizes

This article describes the fabrication of polymer structures with lateral dimensions in the sub-micron regime using hard X-rays (λ_c ≈ 0.4 nm) from the electron storage ring ANKA. Spincoated polymethylmethacrylate (PMMA) grades have been analyzed with respect to development rates and contrast. The contrast has been determined to be constant over a wide dose regime but rapidly decreases for dose values below 1 kJ/cm³. Films with a thickness from 2 to 11 μm have been patterned using a high resolution X-ray mask consisting of 2 μm thick gold absorbers on a suspended 1 μm thick silicon nitride membrane. The fabrication of sub-micron X-ray lithography structures with feature sizes down to 400 nm is confined by the mechanical parameters of the resist material and the process conditions. Surface tension after development limits the achievable aspect ratio of isolated pillars and walls, depending on the actual resist height. PMMA structures have been successfully used as template for electroplating of 1 μm thick gold to demonstrate the fabrication capability of sub-micron scale metal parts.     

Influence of mask substrate materials on resist sidewall roughness in deep X-ray lithography

Absorption of X-rays in deep X-ray lithography masks can significantly increase exposure time, harden the X-ray spectrum and lead to heating and distortion of the mask. To reduce the impact of such absorption, we have evaluated low atomic mass (low-z) materials that allow the utilization of thick substrates (>100 μm) for reliable mask fabrication. Various forms of graphite, vitreous carbon (VC), boron nitride and beryllium were chosen for testing. Transmission tests were conducted to evaluate resulting surface roughness in the X-ray resist sidewalls. We found that VC, beryllium and pyrolytic graphite all have minimal effect on the resist sidewall surface roughness; however, graphite and boron nitride both significantly increase the roughness to about 300 nm RMS. We could show that this increase in surface roughness is directly related to the crystal structure of these materials. From the tests conducted, VC proves a promising mask substrate, superior to the more expensive and hazardous beryllium that is commonly used for thick high precision masks. VC has been successfully employed as a mask substrate and corresponding resist structures are introduced.     

MODULIGA: The LIGA process as a modular production method-current standardization status in Germany

The LIGA process combines X-ray lithography with electroplating and molding. It is a technique used worldwide for the fabrication of high aspect ratio microstructures. In Germany, this technique will be established as a production line to manufacture micro system devices. A project named (modular production method using the LIGA process) MODULIGA has the goal to link together the activities of the German LIGA centers Forschungszentrum Karlsruhe / Institut für Mikrostrukturtechnik, Karlsruhe (FZK/IMT), Institut für Mikrotechnik, Mainz (IMM) and Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung, Berlin (BESSY). LIGA process steps will be standardized and interface specifications will be defined to establish a modular production method. Interface specifications are defined for masks, substrate and resist, exposure technology, development and finally electroplating. The standards are defined by testing criteria. The challenges and the current status of standardization will be described.     

Movable microstructures made by a sub-micron LIGA process

 Movable microstructures with high aspect ratios were made with lateral dimensions down to the sub-micron domain by the LIGA process and a sacrificial layer technique. Compared to microstructures usually made by LIGA, all dimensions were reduced approximately by a factor of 10, while the aspect ratio was kept constant. The smaller resist thickness in the range of some ten micrometers allowed much lower X-ray doses and energies to be used for exposure and the absorbers with a thickness of only 3 μm to be employed. As the lateral dimensions are smaller, a much larger number of devices can be fabricated in one batch. Therefore, the production costs of deep etch X-ray lithography are reduced dramatically. Electrostatic linear actuators with lateral dimensions as small as 500 nm were manufactured to demonstrate the advantages of LIGA in sub-micron dimensions. An X-ray mask with absorbers 2.8 μm high was produced by a three-level technique. The linear actuators consisted of several arrays of capacitor plates combined into an electrostatic driving unit with an active area, typically, 0.3 mm² and less. The driving unit was supported by folded beam flexures to avoid frictional forces. They also guaranteed parallel movement of the capacitor plates. The functionality of these devices was demonstrated by measuring displacement as a function of the voltage applied. Received: 30 October 1995 / Accepted: 17 January 1996     

Movable microstructures made by a sub-micron LIGA process

Movable microstructures with high aspect ratios were made with lateral dimensions down to the sub-micron domain by the LIGA process and a sacrificial layer technique. Compared to microstructures usually made by LIGA, all dimensions were reduced approximately by a factor of 10, while the aspect ratio was kept constant. The smaller resist thickness in the range of some ten micrometers allowed much lower X-ray doses and energies to be used for exposure and the absorbers with a thickness of only 3 μm to be employed. As the lateral dimensions are smaller, a much larger number of devices can be fabricated in one batch. Therefore, the production costs of deep etch X-ray lithography are reduced dramatically. Electrostatic linear actuators with lateral dimensions as small as 500 nm were manufactured to demonstrate the advantages of LIGA in sub-micron dimensions. An X-ray mask with absorbers 2.8 μm high was produced by a three-level technique. The linear actuators consisted of several arrays of capacitor plates combined into an electrostatic driving unit with an active area, typically, 0.3 mm² and less. The driving unit was supported by folded beam flexures to avoid frictional forces. They also guaranteed parallel movement of the capacitor plates. The functionality of these devices was demonstrated by measuring displacement as a function of the voltage applied.     

Microfabrication of thick tungsten films for use as absorbers of deep X-ray lithography masks

 We fabricated thick (5 μm) tungsten (W) film patterns by sputtering and dry etching, and realized a new deep X-ray lithography mask. The X-ray mask with 5-μm-thick W absorbers could expose about 1-mm-thick resist structures. In the deposition process of W films, the column structure of about 0.2 μm grain size, from which pattern edge roughness originates, disappeared by adding nitrogen into the sputtering gas. W film etching was carried out by reducing gas pressure and cooling the substrate (−40 °C), and a side etch width of below 0.2 μm was obtained. From the results of the pattern edge roughness and the side etch width, a pattern fabrication accuracy below ±0.5 μm was achieved. Furthermore, film stress, which induces pattern distortion, was reduced to below 50 MPa by controlling the sputtering gas pressure. The obtained mask achieved a pattern distortion below ±0.3 μm. Received: 7 July 1999/Accepted: 29 May 2000     

A new UV sensitive positive resist for X-ray masks manufacture

LIGA is a well-established process to fabricate metallic micro parts with high resolution, high precision and very low sidewall roughness by means of X-ray lithography and electroplating. The availability of a precise X-ray mask is a precondition for the final precision of the manufactured micro parts. Typical mask substrate materials, e.g. beryllium, carbon based foils, Si₃N₄ or SiC show different disadvantages such as low X-ray transparency or high toxicity or high prices or low conductivity or high thermal expansion or surface porosity causing X-ray scattering. For the fabrication of X-ray masks, PMMA with its unique features such as high aspect ratio patterns with high precision, exhibits low sensitivity and the layers preparation is not easy. SU-8, an epoxy-based UV and X-ray sensitive, chemically amplified, negative tone photoresist exhibits high aspect ratio patterns with vertical sidewalls. The difficult remove of the resist after the electroplating process significantly hinders the inspection of the fabricated X-ray mask. We present the use and suitability of an UV sensitive, chemically amplified, viscous, aqueous-alkaline developable, and easy removable positive tone photoresist, XP mr-P 15 AV, exhibiting high aspect ratio patterns with vertical sidewalls for the fabrication of X-ray masks by means of UV lithography on vitreous carbon substrates.     

Inexpensive, quickly producable X-ray mask for LIGA

 The capability to produce X-ray masks inexpensively and rapidly is expected to greatly enhance the commercial appeal of the LIGA process. This paper presents a process to fabricate X-ray masks both inexpensively (under $1000) and rapidly (within a few days). The process involves one UV lithography step and eliminates the need for an intermediate X-ray mask. The X-ray mask produced by this process consists of a 125 μm thick graphite membrane that supports a gold-on-nickel absorber pattern. The thickness of the absorber structures is great enough to supply sufficient contrast even when radiation sources with high characteristic photon energies up to 40 keV are utilized and/or when deep exposures are desired. The mask fabrication process is initiated by spin coating 30–50 μm of SU-8 directly on a graphite membrane. The SU-8 is then patterned using a UV mask. Gold-on-nickel absorber structures are electroplated directly onto the SU-8 covered graphite. Once the remaining SU-8 is removed, attaching the graphite membrane to a frame completes the mask. To test the performance of the mask, a nickel mold insert was fabricated. A sheet of PMMA 500 μm in thickness was bonded to a nickel substrate, then exposed to X-rays through the mask, and developed. Electroplating nickel into the patterned PMMA sheet produced a mold insert. SEM pictures taken of the SU-8, the X-ray mask, and the mold insert are shown. This method of rapidly producing an inexpensive X-ray mask for LIGA resulted in a mold insert with smooth, vertical sidewalls whose dimensions were within two micrometers of the UV mask dimensions. Received: 12 December 1998/Accepted: 2 February 1999     

Numerical simulation of thermal distortions in deep and ultra deep X-ray lithography

 One major process step in deep X-ray lithography is the exposure of the resist with synchrotron radiation. High energy photons are absorbed in mask, resist and substrate. About 95% of this energy is deposited as thermal heat [Schweizer (1997)]. This may lead to a temperature rise in the system and result in thermal distortions during the patterning process. A sample layout is used to determine the distortions during irradiation. Typical radiation parameters of the ELSA storage ring at Bonn University (2.7 GeV, 35 mA) and material properties are applied to simulate the heat effects. Mask membranes made of titanium or beryllium are modeled to irradiate PMMA layers of 200 and 2500 μm thickness. Copper is used as substrate material. Mask support and the bottom of the substrate are cooled to 21 °C as the system is scanned through the synchrotron beam. In the case of 200 μm PMMA and titanium mask membranes, mask temperatures increase to 40.1 °C, whereas only 22.3 °C are reached if beryllium masks are simulated. Maximum distortions are 0.74 μm for Ti-masks and 0.03 μm for Be-masks. With increasing resist thickness, the incident synchrotron radiation power as well as the temperature rise are reduced. In the case of 2500 μm thick PMMA, temperatures of 21.45 °C are simulated. Received: 10 August 2001/Accepted: 24 September 2001 This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

Fabrication of HARM structures by deep-X-ray lithography using graphite mask technology

The cost-effective fabrication process for high-aspect-ratio microstructures using X-rays depends largely on the availability and quality of X-ray masks. The fabrication of X-ray masks using commercially available graphite sheet stock, as a mask membrane is one approach that is designed to reduce cost and turnaround time. Rigid graphite offers unique properties, such as moderate X-ray transmission, fairly low cost, electrical conductivity, and the ability to be used with either subtractive or additive processes [1, 2]. This paper will demonstrate the potential of a cost-effective, rapid prototyping of high-aspect-ratio microstructures (HARMs) using graphite masks. The graphite wafer accommodates both the intermediate mask and the working mask. In order to allow a direct comparison of the graphite mask quality with other X-ray masks, the primary pattern was derived from a Ti X-ray mask using soft X-ray lithography (XRL). Received: 7 July 1999 / Accepted: 29 September 1999     

Proton beam writing: a tool for high-aspect ratio mask production

P-beam writing (proton beam writing), a direct write 3D nano lithographic technique has been employed for the production of X-ray masks in a single step fabrication process, with high aspect ratios and extremely smooth absorber edges. P-beam writing employs a focused MeV proton beam scanned in a predetermined pattern over a resist (e.g. PMMA or SU-8), which is subsequently chemically developed. P-beam writing in combination with electroplating appears ideally suited to directly write X-ray masks with nano sized features, high aspect ratios, small lateral feature sizes, and smooth and vertical sidewalls. Sub 100 nm resist structures with aspect ratios up 160 have been produced, as well as metallic (nickel) structures down to the 100 nm level. Preliminary tests on p-beam written X-ray test masks show that Ni stencils can be fabricated with a thickness of 2–20 μm, smooth side walls, feature details down to 1 μm, and aspect ratios up to 20.     

Silicon IC compatible LIGA mask-fabrication process

 This paper describes a technique for fabricating a LIGA mask that offers good compatibility with the silicon IC process. X-ray exposure can be eliminated from the LIGA mask-fabrication process, so that LIGA masks can be fabricated with existing silicon IC process equipment. A gold absorber pattern, 2 μm wide and 10 μm thick, has been successfully fabricated by combining a three-layer resist-patterning process with the electroplating process. Improvements in both the mask structure and fabrication process alleviate the problems of dust in a cleanroom and contamination in the etching chamber. Received: 25 August 1997/Accepted: 23 September 1997     

Fabrication of microgratings on PMMA plate and curved surface by using copper mesh as X-ray lithography mask

We demonstrate experimentally the X-ray lithography technique to fabricate microgratings on a PMMA plate and on curved surfaces such as PMMA cylinder lens surfaces with X-ray lithography by copper mesh as mask. Some gratings with 12.7 μm pitches on the plate and on PMMA curved surface with large area (10 mm × 10 mm) by vertically moving or rotating the resist stage exposure are realized.     

Shape deviations in masks for optical structures produced by electron beam lithography

E-beam lithography is a well-known technology used in the structuring of resist for mask fabrication. In the LIGA process E-beam lithography is used to fabricate the first X-ray mask. Due to the high precision of X-ray lithography patterning errors and defects are transformed into the several hundred micrometers thick resist structures. The side walls of these high-aspect-ratio structures are often used as optical mirrors, for which very good surface properties are essential. Deviations in the shape of the side walls even far below the wavelength of the used light lead to spurious strayed or misguided light. For grating microspectrometers the requirements are particularly stringent. The width of the grating teeth does not only have to be precise, but also the grating teeth have to be positioned accurately along the entire grating width. Using LIGA microspectrometers as an example, deviations in masks and LIGA-structured side walls were studied and subsequent correlations made with the corresponding e-beam writing pattern.     

Direct, high throughput LIGA for commercial applications: a progress report

Direct LIGA; LIGA without injection molding; has the potential to become a cost effective, high throughput form of LIGA. The process requires high energy photons; near 20,000 eV; which are best produced in facilities such as the X-ray ring at Brookhaven National Laboratory. The increased absorption lengths over lower energy photons eliminates the need for a membrane type X-ray mask. This in turn facilitates very large area X-ray masks fabricated from standard silicon wafers with 20 μm gold absorbers. The absorption length increase in PMMA to 2 cm is used to implement stacked PMMA exposures in which 1 mm thick PMMA layers are used to produce exposed PMMA sheets. These sheets are eventually solvent bonded to working substrates with plating bases. New high energy X-ray masks have been developed. Two exposure stations at Brookhaven are operational. The recently commissioned manufacturing exposure station which uses a 22 inch scanner which can expose four separate PMMA-mask combination is in the testing phase. Received: 7 July 1999/Accepted: 30 July 1999